Lubricants have been used in internal combustion engines, automatic transmissions or bearings such that they move easily and smoothly. Particularly, lubricating oils for internal combustion engines, i.e., engine oils are required to possess high degree of performances due to higher performances and higher output power of recent internal combustion engines than ever and severe conditions where they run. Therefore, conventional engine oils are blended with various additives such as anti-wear agents, metallic detergents, ashless dispersants, and antioxidants so as to satisfy such requirements. Since the use of such conventional engine oils increases the energy loss due to the friction occurring at some parts of an engine where the oils work, there have been also used fuel efficient lubricating oils containing friction modifiers for reducing the friction loss and fuel consumption. The friction modifiers include oil soluble metallic friction modifiers containing a metal element such as molybdenum and ashless friction modifiers leaving no ash even though it burns. The molybdenum-based friction modifiers are excellent in friction reducing effect when they are fresh but are limited in maintaining the effect for a long period of time with conventional techniques. The molybdenum-based friction modifiers are demanded to be decreased because they adversely affects the exhaust gas purifying device of an internal combustion engine and molybdenum can be an element hindering the recycle of lubricating oils containing the friction modifiers. Whereas, ashless friction modifiers, such as ester-, amine-, or amide-based friction modifiers are free from such drawbacks and have increased in their importance from the viewpoint of environment protection. However, the ashless friction modifiers are much poorer in the friction reducing effect when they are fresh, than the molybdenum-based friction modifiers and thus have been demanded to be improved in the performance capability.
Furthermore, from the view point of recent environmental issues, lubricating oils have been demanded to be improved in the long-drain capability to extend the drain intervals. It is now found that zinc dialkyldithiophosphate (ZDTP) which has been used suitably as an anti-wear agent and an antioxidant is not suitable with the objective of oxidation stability and hydrolytic stability under the recent situations where a further improvement in long drain capability is highly demanded.
The inventors of the present invention discovered that lubricating oil compositions containing less or no ZDTP having been used for many years and containing a specific phosphorus compound was able to exhibit extremely excellent long drain capability (oxidation stability, base number retention properties, and thermal stability) and low friction properties while maintaining anti-wear properties equivalent to those of a lubricating oil containing zinc dithiophosphate and has already filed patent applications for these compositions (Japanese Patent Laid-Open Publication Nos. 2002-29427.1 and 2004-83751).
It was confirmed that when these lubricating oil compositions containing a specific phosphorus compound were optimally blended with other additives, they exhibited anti-wear properties determined by a valve train wear test conducted for domestic engines typically as described in JASO M328-95, equivalent to the anti-wear properties of a composition containing ZDTP. However, a lubricating oil is required to possess extreme pressure properties and anti-wear properties more excellent than ever so as to be used in a special engine driven under more severe conditions or used under particular circumstances where more excellent extreme pressure properties and anti-wear properties are required; or to fulfill a requirement that the phosphorus content is decreased to 0.08% by mass or less to meet the suitableness for an exhaust-gas purifying catalyst in the forthcoming ILSAC GF-4 standard or another requirement of low phosphorus content that the phosphorus content is decreased to 0.05% by mass or less to be sought in ILSAC GF-5 standard which is a plan under consideration. The lubricating oil is demanded to be further improved in the low friction properties.
However, it is difficult to decrease the phosphorus content of a lubricating oil containing an organic molybdenum compound with excellent extreme pressure properties and low friction properties because the compound generally contains phosphorus and/or sulfur. It is also difficult to decrease the phosphorus content of a lubricating oil with the use of a sulfur-free phosphorus compound because the extreme pressure properties and anti-wear properties are deteriorated if the phosphorus content is simply decreased. Alternatively, an increase in the amount of a sulfur-containing compound or of a metal-containing compound adversely affects an exhaust-gas after-treatment device, i.e., fails to solve the problems that an exhaust-gas purifying catalyst such as a ternary catalyst, an oxidation catalyst and a NOx adsorber and a DPF or an exhaust-gas treatment system which is the combination of a DPF with the exhaust-gas purifying catalyst, particularly the oxidation catalyst or NOx adsorber undergo to catalyst poisoning and/or clogging of the DPF caused by the increased sulfur and metal. Furthermore, the lubricating oil will be extremely deteriorated in oxidation stability, base number retention properties and detergency due to the increase in the amount of sulfur and metal.
Therefore, it has been very difficult to produce a lubricating oil which has both anti-wear properties and low friction properties and can be decreased in phosphorus and sulfur contents or additionally ash content while maintaining excellent long-drain capability. A solution of such problems has been demanded.
As described above, ZDTP has been used as an anti-wear agent necessarily in an engine oil but has been demanded to be added in a less amount because it adversely affects an exhaust gas purifying catalyst such as a ternary catalyst for an internal combustion engine and can be an element disturbing the recycle of a lubricating oil. There is disclosed a method wherein deterioration of anti-wear properties caused by decreasing ZDTP is compensated using a hydrazide derivative as an anti-wear agent which does not adversely affect an exhaust gas purifying catalyst (see International Publication Pamphlet No. 02/99017).
However, because hydrazides are relatively high in melting point and insoluble or sparingly soluble in oil at room temperature, it is necessary to maintain the hydrazides at a temperature equal to or higher than the melting point thereof and spend a long time in order to dissolve the hydrazides. However, these hydrazide compounds are precipitated when exposed to a low temperature even though they are once dissolved. It is thus difficult to use the hydrazide compounds in a lubricating oil used from low temperatures to high temperatures. It was found that a technique to render the hydrazide compounds oil soluble at low temperature is needed.
It was also found that when a lubricating oil composition containing hydrazides was blended with a conventional level of zinc dithiophosphate, it is less effective in reducing friction and was still able to be improved.
It was also found that a lubricating oil composition containing hydrazides was still able to be improved in high-temperature detergency, capability to prevent copper from eluting from members or parts made of copper of an engine, and anti-wear properties against valve-train wear when the composition is used as a 0W-20 ultra fuel efficient engine oil, a low phosphorus engine oil (the phosphorus content is 0.08% by mass or less), or a sulfur-free long drain engine oil.